Flashing cursor on black screen? The 137GB limit and the 48bit LBA

16 January, 2013

A client in Llanberis called with a curious problem: on starting up, her laptop, an Inspiron 6000, would show a black screen with a flashing cursor at the top left of her screen and then stop. Okay, sounds like maybe some hardware has stopped working or some sort of malware has deleted some files it shouldn’t have. I would need to check the hardware first and then software.

On every non-Apple computer there is a BIOS (Basic Input-Output System), a small computer program on a chip which, whenever you press the on button, checks that certain hardware is working (the Power On Self Test). It checks that there is memory and that it works, that the graphics chip or add-on card is working, that the keyboard and mouse are attached and primes them all ready for running. If there’s a problem, it will beep at you. The number of beeps and whether they’re long or short or the mixture of long and short is used to identify the precise problem. For instance, a recent laptop I looked at had seven long beeps which meant that the graphics chip or motherboard was fried.

The lack of beeps convinced me that at a certain level, the hardware was working but I checked in the BIOS to be sure. I missed it at first but the hard drive was seen as 137GB even though it was a 160GB hard drive. The issue was that the laptop came of age before a thing called 48bit Large Block Addressing and instead used 28bits. Think of it as trying to uniquely identify boxes using three-digit numbers – once you reach 999, you have no more unique numbers. With 48 bits, you can give addresses to more boxes. When data piled up and over the 137GB limit, the hardware got confused and simply could not address the amount of space that was in use and so stopped.

The answer was to split the hard drive into two sections, one for Windows’ system files and another for the user’s files. The BIOS then saw the hard drive as two separate drives and quite happily booted up.

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